In U.S. Pat. No. 4,519,814 (Demarest) there is disclosed a two-stage glass batch liquefaction process wherein glass batch is preheated by contact with exhaust gas from a liquefying stage to which the preheated glass batch is fed. Such an arrangement entails a relatively small exhaust gas volume in the batch preheating stage, particularly when the combustion heat sources are fired with oxygen instead of air, so that direct contact of the exhaust gas stream with dry, pulverulent batch materials is feasible with relatively little entrainment of particulate material by the exhaust gas stream. Recovering waste heat directly into the batch materials is highly advantageous for the sake of reduced plant construction cost and more efficient heat recovery in comparison with a conventional regenerative furnace. However, a small amount of particulate entrainment usually occurs, which can readily be recovered with conventional particulate separating means without significantly detracting from the advantages of the direct heat recovery system. Disposal of the collected particulates can be a problem due to environmental concerns and the rising costs of waste disposal. Recycling the particulates back into the batch mixture is an attractive approach to solving the waste disposal problem, especially when it is considered that the loss of entrained material can slightly altar the composition of the batch mixture and that some raw material cost savings can be realized by recycling the particulates. Such approach is shown in U.S. Pat. No. 3,880,629 (Dulin et al.), U.S. Pat. No. 4,208,201 (Rueck) and U.S. Pat. No. 4,349,367 (Krumwiede). Unfortunately, the fine particles are easily entrained in the exhaust stream again, so that recycling the collected particulates can worsen the entrainment problem and increase the load on the particulate recovery system.
U.S. Pat. No. 3,030,094 (Saeman) and U.S. Pat. No. 3,508,742 (Minigishi) show glass batch melting arrangements that include preheating, but make no provision for handling particulates. U.S. Pat. No. 4,113,459 (Mattmuller) shows means to separate dust from a glass batch preheater, but makes no disclosure regarding disposal of the collected dust particles. U.S. Pat. No. 4,185,984 (Kiyonaga) discloses preheating glass batch and feeding portions of the batch separately to a melter, but does not deal with particulates lost from the preheated stage.